Induction heating process



g- 19, 1969 1 A. F. LEATHERMAN 3,462,336

INDUCTION HEATING PROCESS Original Filed Oct. 21, 1965 5 Sheets-Sheet 1COOLANT 7 SUPPLY HIGH FREQUENCY INVENTOR.

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INVENTOR.

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INDUCTION HEATING PROCESS Original Filed Oct. 21, 1965 5 Sheets-Sheet 5INVENTOR.

A. F. 45.4 THEE/IAN Arron/n5 United States Patent O 3,462,336 INDUCTIONHEATING PROCESS Alfred F. Leather-man, Columbus, Ohio, assignor, bymesne assignmentl, to William 'C. Heller, Jr. Original application Oct.21, 1965, Ser. No. 499,150, now Patent No. 3,396,258, dated Aug. 6,1968. Divided and this application May 17, 1968, Ser. No. 730,185

Int. Cl. B29c 27/04 US. Cl. 156-272 21 Claims ABSTRACT OF THE DISCLOSUREA process for inductively heating non-metallic material includes thestep of applying a susceptor to the material. An induction heater isprovided by forming :a coil for producing a magnetic field whenenergized and spacedly positioning a non-magnetic, metallic member fromthe coil in the magnetic field. The material is placed between the coiland the metallic member and the heater energized to produce a highfrequency high intensity magnetic field to'inductively heat thesusceptor and heat the material by conduction.

CROSS REFERENCE TO RELATED APPLICATION This application is a divisionalapplication of my c pending application, Ser. No. 499,150, filed Oct.21, 1965, now issued as U.S. Patent No. 3,396,258 on Aug. 6, 1968.

BACKGROUND OF THE INVENTION This invention relates to induction heatingprocesses and more particularly to :a process providing superioroperating efficiencies.

Induction heating is a thermal process in which in its well known forms,electric energy in the form of a high intensity high frequency magneticfield is applied to a metallic substance. The field produces eddycurrents, or hysteresis losses, which cause heat to be generated in thesubstance itself. This method has been in common use for melting andheat treating metals for a number of years.

Induction heating may also be used in the thermal processing ofnon-metallic materials, such as plastics, by placing inductivelyheatable substances, as for example, certain metal or metal oxidestructures or particles at points in the material where heat is desired,and then placing the composite structure in a magnetic field. Forexample, if it is desired to join two sheets of plastic, such aspolyethylene, fine metal or metal oxide particles, or a metal screen,may be placed between the sheets at the points desired to be joined.When a magnetic field is applied to sheets, the particles or screenbecomes heated, softening the plastic and allowing the two sheets tofuse. The metallic particles or structure is generally termed asusceptor to indicate its capability of being heated by a magneticfield.

The above method of thermal processing differs from dielectric thermalprocessing in which the non-metallic substances is itself heated by ahigh frequency electric field. Dielectric thermal processing involvesconsiderations not here pertinent.

The advantages of such thermal process includes the fact that heat isgenerated only at the location where it is to be used, thereby providingideal temperature distribution which permits accurate and beneficialcontrol of temperature. Additionally, since heat is not required to flowfrom an external source through the material to the desired location,substantial increases in the rate of thermal processing are obtainable.The accurate temperature control and shortened exposure times preventthermal damage, such as charring, warping or distortion from occurringduring the processing.

It is necessary, in order to obtain the above advantages in commerciallyand technically feasible process, such as the heat sealing of plastics,to provide a magnetic force, or field, of the highest possible intensityand of the highest possible frequency so as to generate the greatestamount of heat by induction losses.

The equipment used to generate such a field generally consists of afield producing apparatus (e.g. work coil) coupled to a high frequencypower source. The attainment of both of the aforementioned criteriadepends to a great extent upon the proper utilization of this apparatusin the process.

An excessive amount of inductance in the apparatus limits the magnitudeof the high frequency current flowing through the apparatus and hencethe intensity of the magnetic field generated thereby. While a greaterapplied voltage may be used to increase current flow, this introducesinefiiciencies to the process. The size of the field producing apparatusmay also be reduced to lower the inductance but often only at theexpense of a decrease in processing speed or capacity.

Further, it is generally desirable to conduct the process with theapparatus in parallel resonance with the power source, as current flowat such a frequency is maximized. Resonant frequency is determined bythe formula A low value of inductance permits the resonant frequency ofthe apparatus to be high enough to generate induction losses of therequired magnitude. For example, a resonant frequency of 4 megacyclesmay be required in thermal processing non-metallic materials. This issignificantly above the frequencies required for metallurgical useswhich generally range from 3 kilocycles to 450 kilocycles.

It is also desirable to direct or channel the magnetic field of theapparatus so as to concentrate as much of it as possible in the materialbeing thermally processed. Flux not so utilized performs no usefulfunction and, unless cancelled out or reduced, increases the inductanceof the apparatus unnecessarily.

SUMMARY It is the object of this invention to provide an improvedprocess of inductively heating non-metallic materials which processprovides accurate temperature control at high processing speeds withoutthe risk of thermal damage to the material, such process beingparticularly adapted for use in sealing or uniting non-metallic webmaterials.

In the performance of the process of the present invention, a susceptorsusceptible to heating by a high fre quency magnetic field is applied tothe portions of the non-metallic material to be heat treated.

An induction heating apparatus is provided by forming a field producingcoil and spacedly positioning a nonmagnetic metallic member in themagnetic field of the coil. The metallic member is energizable when thecoil is excited to concentrate the magnetic field in the space betweenthe member and the coil and to reduce the inductance of the heatingapparatus.

The material is placed between the coil and the member. The magneticfield inductively heats the susceptor which, in turn, heats thenon-metallic material by conduction to provide the desired heattreatment.

3 BRIEF DESCRIPTION OF THE DRAWING The invention may be betterunderstood by reference to the following specification and drawings,forming a part thereof, in which:

FIGURES la and lb show an induction heating apparatus;

FIGURE 2 shows one embodiment of an improved low inductance heatingapparatus and the process of the present invention;

FIGURES 3a, 3b, 3c, and 3d show another embodiment of the process of thepresent invention particularly adapted to the sealing of articles madefrom non-metallic materials; and

FIGURES 4a, 4b, 4c, and 4d show a further embodiment ofthe process ofthe present invention particularly adapted to seal the periphery of aplastic sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the specification anddrawings, the utility of the present invention is demonstrated byshowing its application in sealing plastic or plastic coated structuresby inductively heating a susceptor placed between the plasticstructures. It is to be understood that the use of the invention is notso limited. In addition to the aforementioned plastic, the process maybe used to advantage with cardboard stock, wood, rubber, glass andpaper. The process also finds utility in many fields of manufacture.

Referring now to the figures, and specifically to FIG. In, there isshown therein an induction heat sealing apparatus 2. The apparatusincludes a coil member 4. While coil member 4 may comprise any number ofloops, the lowest value of inductance is obtained with a coil of asingle loop having the smallest enclosed space. Hence, coil 4 as shownin FIG. 1a is shaped in the form of a single loop or hairpin with twoparallel members 4A and 4B. As is customary in the art, coil 4 maycontain cavity 6 for the conduction of a cooling fluid therethrough.Coil 4 is connected to a coolant supply 7 and a high frequency powersource 22 by connectors 8. High frequency power source 22 may, forexample, provide coil 4 with power of 4 megacycle frequency. Coil 4 maybe supported and insulated by electrically non-conducting support 10.

It will be appreciated that when high frequency electric current isapplied to coil 4 from source 22 a magnetic field will be generatedwhich will surround each of the parallel members of coil 4. While thismagnetic field will exist at all points about each of the members ofcoil 4, the point of greatest intensity will be between the members ofcoil 4. Assuming the instantaneous direction of the high frequencycurrent to be into the plane of the paper in loop member 4B, as shown bythe tail of the current arrow, and out of the plane of the paper in loopmember 4A, as shown by head of the current arrow, the direction of themagnetic field between the members of coil 4 is shown by arrows 12. Thedirection of this field will, of course, reverse as the direction ofcurrent in the members of loop 4 reverses.

When two plastic sheets 14 and 16 having a susceptor 18 between them areplaced between members 4A and 4B of coil 4, the susceptor will becomeheated by the aforementioned mechanism of eddy current and/or hysteresisheating. This heat will be transferred to the adjacent internal surfacesof plastic sheets 14 and 16 softening them and permitting them to becomefused or sealed upon removal of the sheets from between members 4A and43. Such a process may be used to form the corner seam of a plasticpackage.

While the coil structure shown in FIG. 1a and FIG. 1b is one ofrelatively low inductance since it consists of a single loop, theinductance may still not be sufficiently low to provide the desiredresonant frequency or field strength. Additionally, the magnetic fieldgenerated by coil 4 is of greatest intensity only between coil members4A and 43 rather than throughout the entire area covered by susceptor18.

FIG. 2 shows an induction heating apparatus coil which provides a lowinductance and a concentrated magnetic field. Similar to the coil shownin FIGS. 1a and 1b, the coil of FIG. 2 contains a pair of parallelmembers 4A and 4B supported in support 10. In addition, the coil of FIG.2 includes an electrical reinforcing" member 20, so termed because itreinforces or concentrates the magnetic field generated by coil members4A and 4B. Reinforcing member 20 is located between coil member 4A andcoil member 4B and may be wedge shaped in form. Other shapes may be usedif desired. Plastic sheets 14 and 16 and susceptor 18 are placed betweencoil members 4A and 4B and reinforcing member 20. Upon connecting coil 4to a source of high frequency electrical energy 22, a current will beinduced in reinforcing member 20 by the magnetic field generated aroundcoil 4. The direction of current flow in reinforcing member 20 will bein an opposite sense to that of coil member 4 as shown by the head andtail of arrows for an instantaneous case in FIG. 2. The development ofthis current flow and its polarity in reinforcing member 20 is describedby the physical principles of Lenzs law.

The current flow in reinforcing member 20 will produce a magnetic fieldabout that member shown schematically by the numeral 13. The portion ofmagnetic field 12 formerly present in the area occupied by reinforcingmember 20 will be neutralized by the action of the current induced inreinforcing member 20. Specifically, this is due to the opposition ofmagnetic field 13 (created by the induced currents) to magnetic field12. This eliminates the portion of the field 12 generated by coil 4 thatwas useless and unneeded in the sealing process since it was remote fromthe location of susceptor 18. The elimination of this flux alsosignificantly reduces the inductance of coil 4. In addition, and equallyimportant, the intensity of the magnetic field immediately betweenreinforcing member 20 and each of the coil members 4A and 4B isincreased, as the field 13 generated by reinforcing member 20 in thisregion reinforces the field 12 created by coil 4. This increase, ofcourse, is highly desirable as it occurs in the region required forinduction heat treating of plastic sheets 14 and 16. The intensity ofthe magnetic field in the aforementioned space may be increased up to atheoretical maximum of twice the intensity present at the correspondingpoint in FIG. 1b as a result of the reinforcement of magnetic field 12by magnetic field 13.

In electrical construction, reinforcing member 20 may be electricallyisolated, or grounded, as desired. Reinforcing member 20 should beconstructed of a good electrical conductor, such as silver, copper, orbrass. From the mechanical standpoint, reinforcing member 20 should be agood thermal conductor and hence a construction of solid silver, copperwith silver plating on the surfaces adjacent coil members 4A and 4B,solid copper, or brass with silver or copper plating on the surfacesadjacent coil members 4A and 4B provide a good combination of bothelectrical and mechanical requirements. If necessary, reinforcing member20 may be cooled by convection, conduction or forced cooling. Thus,member 20 may be solid and air-cooled, solid with liquid coolantcirculating tubes attached thereto, or hollow with internal fluidpassages similar to those of coil member 4. Cooling of reinforcingmember 20 serves to maintain the mechanical and electricalcharacteristics of the member.

By adequate cooling of reinforcing member 20 further advantages of thepresent invention may be obtained. Not only may reinforcing member 20provide the aforementioned reduction in external impedance of coilmember 4 and an increase in the useful magnetic field, it may also beused as a pressure element or positioning element for plastic sheets 14and 16 and susceptor 18. For instance, fiat plastic sheets 14 and 16 maybe placed on support member with susceptor 18 between them and bent tothe required position shown in FIG. 2 by the insertion of a wedge shapedreinforcing member 20 into support 10. During the time susceptor 18 isheated by the magnetic field generated by coil member 4 and reinforcingmember 20, the latter member may be used as a pressure element inconnection with support member 10 to insure greatest possible contactbetween plastic sheets 14 and 16 and susceptor 18. When the requireddegree of heat has been produced in susceptor 18 and the adjacentsurfaces of sheets 14 and 16, high frequency source 22 may bedisconnected from coil 4 and reinforcing member 20 and the coil, cooledby the aforesaid means, used to rapidly solidify the heated portions ofplastic sheets 14 and 16. This also increases the speed with which thethermal processing may be performed.

FIG. 3 illustrates an induction heating process, incorporating theprinciples outlined in FIG. 2, which is suitable for induction heatsealing the two halves of a plastic article. This may, for example, becontainer 24 as shown in FIG. 3a. Coil 5 having members 5A and 5B isarranged in a loop, the configuration of the loop conforming roughly tothe cross sectional shape of container 24. Coil 5 is mounted in asupport member 26 having a hole in the center thereof which correspondsto the cross sectional configuration of container 24. Coil S is suppliedwith high frequency alternating current and a coolant for a hollowcavity 7 therein through connectors 9. The reinforcing member takes theform of a divided interior member arranged in two halves 2'8 and 30 topermit their removal from container 24 after the two container halves 32and 34 have been sealed. The halves 28 and 30 of the reinforcing memberconform to the inner configuration of container 24 and include matingdiagonal edges 42 and 44. Reinforcing members 28 and 30 contain passages36 through which a cooling medium supplied by pipes 38 may be circulatedto cool the members.

The two halves 32 and 34 of container 24 are assembled adjacent coilmember 5 with magnetically heatable susceptor 18 between the halves. Theassembly is retained in position by positioning reinforcing members 28and 30 as shown in FIG. 3b. High frequency power is then supplied tocoil 5 which induces heat in susceptor 18 softening the portions ofcontainer halves 32 and 34 in contact therewith. Reinforcing members 28and 30 operate in the same manner as reinforcing member 20 toconcentrate the magnetic field produced by coil member 5 along theoverlapping edges of container halves 32 and 34. Pressure may be appliedto the halves of container 24 by a force exerted on the exposed ends 40of reinforcing members 28 and 30 in the direction of the arrows in FIG.3b. The diagonal edges 42 and 44 provide for an outward extension of thereinforcing members creating the pressure. A downward force may also beprovided on the exposed ends 40 of reinforcing members 28 and 30 to formthe seal on the bottom of container 24. As previously described, whenportions of container halves 32 and 34 have been softened by the heatfrom susceptor 18 and fused from the pressure exerted by reinforcingmembers 28 and 30, power may be removed from coil 5 and the temperaturethereof reduced by the coolant in passages 7 and 36 to hasten thesolidification of the edges of container halves 32 and 34. When thehalves have been joined, the container may be removed from support 26and reinforcing members 28 and 30 extracted from the container bybreaking them apart as shown in FIG. 3c and removing them through theneck of container 24.

It is to be understood that the configuration of container halves 32 and34 as shown in FIG. 3 is not the only one that may be employed to form acontainer 24. For example, container halves 32 and 34 may have abuttingedges with susceptor 18 between them rather than the overlapping edgesas shown in FIGS. 3b and 30. Other configurations may be employed, suchconfigurations beirfig considered well within the knowledge of oneskilled in e art.

FIG. 4 shows a process which may be used to seal the periphery of twoplastic sheets 14 and 16 by means of a susceptor 18, as for example, toform the flat package 48 with the central cavity shown in FIG. 4a. Theprocess employs a coil 4 similar to that described above and areinforcing member 46 which is similar in function and operation toreinforcing member 20 of FIG. 2 and reinforcing members 28 and 30 ofFIG. 3. Reinforcing member 46 may be constructed in the form of thehollow ring shown in FIG. 4b or in the shape of a dish having anupturned edge 47 as shown in FIG. 4d. As above, reinforcing member 46provides induced currents opposing the currents in coil 4. The inducedcurrents in reinforcing member 46 intensify the magnetic field betweenmember 46 and coil 41. Member 46 may also provide pressure and coolingto the plastic sheets forming package 48 in the same manner as thereinforcing members described above. Additionally, reinforcing member 46reduces the inductance of coil 41 by providing a counteracting magneticfield 13 which eliminates unneeded flux 12 in the center portion of coil41. This flux is unneeded, of course, because induction heating ofsusceptor 18 occurs only at the edges of sheets 14 and 16.

It will be appreciated that modifications and embodiments of the abovedescribed invention may be made and it is desired to include all suchembodiments as come within the scope and spirit of the appended claims.

I claim:

1. A process for inductively heating a non-metallic material comprisingthe steps of:

applying a susceptor susceptible to heating by a high frequency magneticfield to the portions of the nonmetallic material to be heated;

providing an induction heater by forming a coil for producing asurrounding high frequency magnetic field when energized, and spacedlypositioning a non-magnetic, metallic member from the coil in themagnetic field to increase the intensity of the field and reduce theinductance of the coil;

placing the material portions having the susceptor ap plied theretobetween the coil and the metallic member; and

energizing the induction heater with high frequency electric current toproduce a high frequency, high intensity magnetic field thereby toinductively heat the susceptor and heat the material by conduction.

2. The process of claim 1 including the step of applying pressure tosaid material by relatively moving the coil and the metallic membertoward each other.

3. The process of claim 1 wherein the material is plastic and thesusceptor is iron oxide.

4. The process of claim 1 further defined as including the step ofcooling the coil to cool the material after it has been heated.

5. The process of claim 4 further defined as including the step ofcooling the metallic member to cool the material after it has beenheated.

6. The process of claim 1 wherein the step of placing the materialbetween the coil and the metallic member is further defined aspositioning the material portions having the susceptor applied theretobetween the coil and the metallic member by means of the metallicmember.

7. The process of claim 1 wherein the step of providing an inductionheater is further defined as forming the coil in a single loop.

8. The process of claim 1 wherein the step of providing an inductionheater is further defined as forming a coil in a single loop with a pairof parallel members for producing a surrounding high frequency magneticfield when energized and spacedly positioning a wedge shapednon-metallic member at least partially between the parallel members andin the magnetic field to increase the intensity of the field.

9. A process for inductively heating non-metallic article portions toform an article by heat sealing the portions at selected locations; saidprocess comprising the steps of:

applying a susceptor susceptible to heating by a high frequency magneticfield to at least one of the article portions at the selected locationsto be sealed;

providing an induction heater by forming a coil in a single loop with anopening to receive the portions, said coil producing a high frequencymagnetic field when energized, and spacedly positioning a nonmagneticmetallic member in the opening and in the magnetic field to increase theintensity of the field and reduce the inductance of the coil;

placing the article portions in the opening of the coil with theselected locations adjacent each other and between the coil and themember; and

energizing the induction heater with high frequency electric current toproduce a high frequency, high intensity magnetic field to inductivelyheat the susceptor and to heat the selected locations by cor1- ductionto seal the portions.

10. The process of claim 9 wherein the article formed is a hollowcontainer and wherein the process is further defined as assembling thearticle portions after applying the susceptor so that the selectedlocations are adjacent each other, positioning the metallic memberinside the hollow article portions when so assembled, and removing themetallic member from the assembled portions after energizing theinduction heater.

11. The process of claim 9 further including the step of applyingpressure to the portions by moving the metallic member toward the coil.

12. The process of claim 11 including the step of applying pressure tothe portions by expanding the dimensions of the metallic member so as tomove it toward the coil.

13. The process of claim 8 further defined as including the step ofcooling the coil to cool the selected locations after they have beenheated.

14. The process of claim 13 further defined as including the step ofcooling the metallic member to cool the selected locations after theyhave been heated.

15. The process of claim 1 wherein the step of providing an inductionheater is further defined as providing a generally planar coil formed ina single loop and shaped to correspond to the portions of thenon-metallic material to which the susceptor has been applied andpositioning a non-magnetic metallic member having a portion presenting agenerally planar surface similar in shape to the coil so that theportion is in the magnetic field and in alignment with, but spaced from,the coil.

16. A process for inductively heating non-metallic article portions toform an article by heat sealing the portions at selected locations, saidprocess comprising the steps of:

applying a susceptor susceptible to heating by a high frequency magneticfield to the article portions at the selected locations to be sealed;

assembling the portions so that the locations are adjacent each other;

providing an induction heater by forming a generally planar single loopcoil shaped to correspond to the selected locations, said coil producinga high frequency magnetic field when energized, and positioning anon-magnetic metallic member having a portion presenting a generallyplanar surface similar in shape to the coil so that the portion is inthe magnetic field and in alignment with, but spaced from, the coil;

placing the assembled portions in the induction heater so that theselected locations are between the coil and the metallic member; and

energizing the induction heater to produce a high frequency, highintensity magnetic field to inductively heat said susceptor and heat thematerial by conduction at the selected locations to seal the portions.

17. The process of claim 16 further including the step of applyingpressure to the portions by relatively moving the metallic member andcoil toward each other.

18. The process of claim 16 further defined as including the step ofcooling the coil to cool the portions at the selected locations.

19. The process of claim 18 further defined as including the step ofcooling the metallic member to cool the portions at the selectedlocations.

20. The process of claim 16 wherein the step of placing the materialbetween the coil and the metallic member is further defined aspositioning the portions by means of the metallic member.

21. The process of claim 16 further characterized in that said articleformed is a container produced by sealing the non-metallic articleportions at locations about the periphery thereof.

References Cited UNITED STATES PATENTS 7/1949 Black 219-1053 2/1968Edwards 156272

